Transformer-coupled power converter sampling system

ABSTRACT

A transformer coupled DC-DC converter wherein the regulation of the output voltage is achieved by sampling the voltage across the transformer primary winding during a particular interval of the cycle. This sampling is achieved by the use of a field effect transistor that is gated on at a time just prior to the time the transformer field collapses. This sample voltage is held by a charged capacitor and used to control a blocking oscillator.

O United States Patent 1191 1111 3,764,881 Thomas Oct. 9, 1973TRANSFORMER-COUPLED POWER 3,629,622 12/1971 Denenberg 323/1310. 1 CONVESAMPLING SYSTEM 3,629,686 12/1971 Hendrikus et a1. 323/DIG. 1 3,671,8426/1972 McKeown 321/2 X [75] Inventor: Robert M. Thomas, Brockville,

Ontario, Canada I Primary ExaminerA. D. Pellinen [73] Assigneez GTEAutomatic Electric Munerheim et aL Laboratories Incorporated, Northlake,Ill.

22 Filed: June 29, 1972 AB Appl. No.: 267,350

A transformer coupled DC-DC converter wherein the regulation of theoutput voltage is achieved by sampling the voltage across thetransformer primary wind- 321/2 ing during a Particular interval of thecycle. This samg i :7 18 pling is achieved by the use of a field effecttransistor 323) that is gated on at a time just prior to the time thetransformer field collapses. This sample voltage is held by a chargedcapacitor and used to control a blocking [56] References Citedoscillator.

UNITED STATES PATENTS 3,564,393 2/1971 Williamson 323/17 6 Claims, 2Drawing Figures YIN (9L BLOCKING X} Tl OSCILLATOR NI {N2 EAITRANSFORMER-COUPLED POWER CONVERTER SAMPLING SYSTEM BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to regulatedvoltage electronic power converters for converting a first DC voltage toa second DC voltage. More specifically, the present invention relates toa control system within such converter, that is simpler and moreeconomical to manufacture.

2. Description of the Prior Art DC to DC converters are well known. Manytypes and variations are to be found in standard reference works. Thesecommonly employ various vibrator circuits using silicon controlledrectifiers, transistors and other devices as switching elements in theinitial conversion of the direct current to alternating current. In asecond conversion the alternating current is rectified at a secondpotential. The second potential is obtained by the use of a transformer.Transformer-coupled converters are used whenever a voltage conversionand/or electrical isolation is required.

Regulation of the DC output voltage is achieved by sampling this voltageand feeding an error signal back to the oscillator or chopper to changeits frequency and/or duty cycle. Power converters using this approachare in common use. An example of such a converter is that disclosed inUS. Pat. No. 3,515,974 issued June 2, I970.

When isolation must be maintained between the input and output, anisolating element must be inserted in the feedback path; this canconsist of a controlled oscillator-transformer-rectifier combination asin the supplies mentioned above, or optical or other coupling can beused. In general, the necessity of providing this isolating element addsto the cost and complexity of the supply.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a converter circuit that does not require a secondisolating element.

In accordance with the invention, the power transformer primary voltageis used to derive a model of the secondary voltage waveform, thuseliminating a second isolating element.

BRIEF DESCRIPTION OF THE DRAWING These features and other advantages ofthis invention will be readily appreciated, as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

FIG. 1 is a schematic circuit diagram of a converter including a voltagesampling system according to this invention; and

FIG. 2 depicts a waveform of the voltage across the primary winding ofthe transformer.

DESCRIPTION OF THE PREFERRED EMBODIMENT General:

The type of converter to which this invention has been applied uses ablocking-oscillator circuit to build up a magnetic field in the powertransformer (FIG. 1); the blocking oscillator shuts off and the magneticfield in the transformer core collapses, causing secondary current toflow via the rectifier into the filter capacitor.

The primary voltage waveform is as shown in FIG. 2; during the off" partof the blocking-oscillator cycle, the voltage induced in the primary is:

V (V u V V (1) where N1 Primary Turns N2 Secondary Turns V DC OutputVoltage V,, Rectifier Forward Voltage V Drop in Secondary CopperResistance V consists of a logarithmic function of secondary currentplus a term I, R where I, is secondary current and R is the diode bulkresistance. The logarithmic function may be closely approximated by aconstant V V is simply equal to 1,, R, where R, is the secondary copperresistance.

From (1), V Nl/N2 (V V I, R,, 1, R At time t I, is small, so V Nl/N2 (VV If a voltage divider of ratio N2/Nl is placed across the primary, itwill deliver a voltage V, V V at time t A voltage V can be subtractedfrom this using the forward drop of a small silicon diode, leaving avoltage a aul- Using a sample-and-hold circuit, a voltage sample V canbe generated and used to control the blocking oscillator to hold V andtherefore V constant.

A circuit according to this concept is shown in FIG. 1. A- gating signalfor the field effect transistor Q2 sample-and-hold circuit is derivedvia the network consisting of resistor R1, capacitor C3 and transistorQ1. This shuts transistor Q2 off at t, when the waveform starts to gonegative.

Capacitor C2 forms a lag network with the divider network, so that thevoltage stored on capacitor C2 is a model of the voltage time t A moreaccurate model could be generated if necessary by using a delay linebetween the voltage divider and the sample-and-hold circuit. Detail.Referring to the drawing there is shown a DC to DC converter astypically used in electronic telephone exchange power supplies,consisting of the direct current chopping or blocking oscillator B01connected in the primary circuit of the step down transformer T1, withthe control circuit C01 control output 2 connected to the blockingoscillator and the voltage sensing leads connected across thetransformer T1 primary winding N1. The secondary winding N2 oftransformer T1 is shown with a rectifier D6 in the series path with theload L. A filter capacitor C4 is shown across the output leads andV,,,,,.

The control circuit C01 input is from the terminals X and Y across theprimary winding N1 of transformer T1. At the time the switch is turnedon in the blocking oscillator B01 the terminal X is negative. Thevoltage divider resistors R5 and R6 connected across these inputconductors do not conduct during the interval that the magnetic field inthe transformer T1 builds up due to the direction the diode D3, inseries with these resistors, is poled. However, a diode D4, serving as abias supply rectifier is connected in series with capacitor C1 acrossthe terminals X and Y, does conduct to charge the capacitor. A currentlimiting resistor R4 in series with a voltage regulating diode D isplaced across the capacitor C1. The negative reference potential at theanode of diode D5 is connected to the base of transistor Q1 to provideit with a constant bias.

As the current flow reverses in the primary winding Nl diode D4 ceasesconduction, and the charge on capacitor Cl is maintained. At this timediode D3 conducts to pass a current through the voltage dividerresistors R5 and R6. The values of these resistors are selected suchthat the voltage across resistor R6 is representative of the voltageacross the secondary N2 of the transformer T1, when the resistance ofresistors R5 and R6 is representative of the primary winding voltage(R6/R5+R6 N2/Nl).

At the time the flow through the primary winding N1 reaches a point intime as shown by the point t on the waveform chart FIG. 2 the voltageacross resistor R6 is gated through. This gating operation is performedby the field effect transistor Q2 which has its source electrodeconnected to the junction of resistors R5 and R6, and its drainelectrode via diode D2 and variable resistor R2 to the input of an erroramplifier EAl.

Diode D2 is used to compensate for the voltage drop of diode D6 in theoutput, and variable resistor R2 is used to adjust the output voltage tothe error amplifier. The delay network, for providing the lag to theerror amplifier, of the sensed voltage V5 is comprised of a capacitor C2in this embodiment. The voltage sample V is that stored on the upperplate of capacitor C2. It is connected from the junction of the drainelectrode of transistor Q2 and the anode of diode D2 to the Y conductor.Capacitor C2 is shunted by the series path consisting of diode D2,variable resistor R2, resistor R3, diode D5. The return path for theinput of the error amplifier EAl is to the conductor of terminal Y,while the output amplified to the proper operating level is connected tothe input of the blocking oscillator.

Transistor Q1 and its associated components function to assisttransistor O2 to turn on and off at the proper times in the cycle.

Transistor O1 is connected with its collector connected to the controlelectrode of the field effect transistor Q2 and via a resistor R1 to theterminal X. The emitter of transistor Q1 is connected to capacitor C3,whose other plate is connected to terminal X. A diode D1 is shown acrosstransistor Q1 with its cathode connected to the base and its anode tothe emitter of transistor O1 to protect it from inverse polarity surges.The collector of transistor Q] is connected to the gate electrode oftransistor Q2 and a terminal of resistor R1. The remaining terminal ofresistor R1 is connected to the terminal X of transformer T1.

Positive potential via resistor R1 to the gate electrode of Q2 normallycauses transistor Q2 to become conductive when terminal X becomespositive. To turn off transistor Q2 the circuit including transistor Q1is added; as the voltage at terminal X starts to drop capacitor C3 isdischarged via the emitter to collector path of transistor Q1 andresistor R1, thus bringing the junction of the gate electrode oftransistor Q2 and resistor Rl to a negative potential much faster thanwould be the case with only resistor R1. In this way, C2 does not loseits charge through R6, and a sample of the output voltage at time 2, isretained.

What is claimed is:

l. A voltage converter for connection between a source of direct currentof a first potential and a load requiring a direct current of a secondpotential, comprising: an oscillator connected to said source andoperated to produce an alternating current output, said oscillatorhaving a control input operative to control said output; a transformerconnected between said oscillator and said load including a primarywinding connected to said oscillator output and a secondary windingconnected to said load; rectification means, connected between saidtransformer and said load, operative to convert said alternating currentpotential to direct currentpotential and to apply it to said load; and acontrol circuit including a first and a second resistor and a diode inseries connected across said transformer primary winding; a gating meansincluding: a field effect transistor having a source, a drain and a gateelectrode, said source electrode connected to the junction of said firstand second resistors, a third resistor, said gate electrode connected toone side of said transformer primary winding via said third resistor,said drain electrode connected to said oscillator control input, saidfield effect transistor conditioned in response to a selected portion ofeach cycle of said oscillator output to pass the potential level at saidjunction of said first and second resistors to said oscillator controlinput, whereby said oscillator is controlled to vary its operation inresponse to the load conditions sensed by said control circuit.

2. A voltage convertor as claimed in claim 1 wherein said controlcircuit further includes an amplifier connected between said drainelectrode and said oscillator control input.

3. A voltage converter as claimed in claim 1 wherein said controlcircuit further includes a transistor having a base, an emitter and acollector, said collector connected to said gate electrode, a capacitor,said emitter connected to said one side of said transformer primarywinding via said capacitor, a voltage bias means, said base operativelyconnected to said voltage bias means, said transistor operated inresponse to a selected portion of each cycle to accelerate the turn offof said field effect transistor.

4. A voltage converter as claimed in claim 1 wherein said controlcircuit further includes a diode in series with said connection to saidoscillator input to compensate for said rectification means voltagedrop.

5. An inverter for connection between a source of direct currentpotential and a load comprising: an oscillator connected to said sourceand operated to produce an alternating current output, said oscillatorhaving a control input operative to control said output; a transformerconnected between said oscillator and said load including a primarywinding connected to said oscillator output and a secondary windingconnected to said load; and a control circuit including a first and asecond resistor and a diode in series connected across said transformerprimary winding; a gating means including; a field effect transistorhaving a source, a drain and a gate electrode, said source electrodeconnected to the junction of said first and second resistors, a thirdresistor, said gate electrode connected to one side of said transformerprimary winding via said third resistor, said drain electrode connectedto said oscillator control input, said field effect transistorconditioned in response to a selected portion of each cycle of saidoscillator output to pass the potential level at said junction of saidfirst and second resistors to said oscillator control input, wherebysaid oscillator is controlled to vary its op eration in response to theload conditions sensed by said control circuit.

6. An inverter as claimed in claim 5 wherein said control circuitfurther includes an amplifier connected between said drain electrode andsaid oscillator control input.

1. A voltage converter for connection between a source of direct currentof a first potential and a load requiring a direct current of a secondpotential, comprising: an oscillator connected to said source andoperated to produce an alternating current output, said oscillatorhaving a control input operative to control said output; a transformerconnected between said oscillator and said load including a primarywinding connected to said oscillator output and a secondary windingconnected to said load; rectification means, connected between saidtransformer and said load, operative to convert said alternating currentpotential to direct current potential and to apply it to Said load; anda control circuit including a first and a second resistor and a diode inseries connected across said transformer primary winding; a gating meansincluding: a field effect transistor having a source, a drain and a gateelectrode, said source electrode connected to the junction of said firstand second resistors, a third resistor, said gate electrode connected toone side of said transformer primary winding via said third resistor,said drain electrode connected to said oscillator control input, saidfield effect transistor conditioned in response to a selected portion ofeach cycle of said oscillator output to pass the potential level at saidjunction of said first and second resistors to said oscillator controlinput, whereby said oscillator is controlled to vary its operation inresponse to the load conditions sensed by said control circuit.
 2. Avoltage convertor as claimed in claim 1 wherein said control circuitfurther includes an amplifier connected between said drain electrode andsaid oscillator control input.
 3. A voltage converter as claimed inclaim 1 wherein said control circuit further includes a transistorhaving a base, an emitter and a collector, said collector connected tosaid gate electrode, a capacitor, said emitter connected to said oneside of said transformer primary winding via said capacitor, a voltagebias means, said base operatively connected to said voltage bias means,said transistor operated in response to a selected portion of each cycleto accelerate the turn off of said field effect transistor.
 4. A voltageconverter as claimed in claim 1 wherein said control circuit furtherincludes a diode in series with said connection to said oscillator inputto compensate for said rectification means voltage drop.
 5. An inverterfor connection between a source of direct current potential and a loadcomprising: an oscillator connected to said source and operated toproduce an alternating current output, said oscillator having a controlinput operative to control said output; a transformer connected betweensaid oscillator and said load including a primary winding connected tosaid oscillator output and a secondary winding connected to said load;and a control circuit including a first and a second resistor and adiode in series connected across said transformer primary winding; agating means including; a field effect transistor having a source, adrain and a gate electrode, said source electrode connected to thejunction of said first and second resistors, a third resistor, said gateelectrode connected to one side of said transformer primary winding viasaid third resistor, said drain electrode connected to said oscillatorcontrol input, said field effect transistor conditioned in response to aselected portion of each cycle of said oscillator output to pass thepotential level at said junction of said first and second resistors tosaid oscillator control input, whereby said oscillator is controlled tovary its operation in response to the load conditions sensed by saidcontrol circuit.
 6. An inverter as claimed in claim 5 wherein saidcontrol circuit further includes an amplifier connected between saiddrain electrode and said oscillator control input.